Efficiently using paper resources is a recent large challenge to protect the global environment and suppress the greenhouse effect caused by CO2. There exist “reuse” techniques for efficient utilization of paper resources. One of the reuse techniques includes printing an image on a paper sheet (image recording medium) using an erasable ink and then heating the paper sheet to erase (decolor) the image. When an image is printed again on the paper sheet that has undergone the image erase, a density change (so-called memory phenomenon) may occur in the erased image portion.